1. Field of the Invention
The present invention relates to an oxygen sensor to be mounted in an exhaust system of an internal combustion engine for the purpose of air/fuel ratio control of the engine, and more particularly, to a device for controlling temperature of such an oxygen sensor.
2. Description of the Prior Art
The oxygen sensor for detecting oxygen in the exhaust gases of an internal combustion engine is considered to be a device indispensable for controlling operation of the engine in a manner of accomplishing power up, fuel economy and exhaust gas purification. The oxygen sensor is generally made of a semi conductive material like a solid electrolyte, and provides an electrical output signal which changes in accordance with changes of oxygen density and also in accordance with changes of its temperature.
For example, the theoretical air/fuel ratio detectable by an oxygen sensor made of titanium dioxide according to whether oxygen exists or does not exist in the exhaust gases of an internal combustion engine changes in accordance with the temperature of the oxygen sensor as shown in FIG. 1. Therefore, if the oxygen sensor should detect air/fuel ratio in a range such as between a.sub.1 and a.sub.2, the temperature of the oxygen sensor must be controlled to be in a range such as between T.sub.1 and T.sub.2. Therefore, it is known to provide a heater for the oxygen sensor for controlling the temperature of the oxygen sensor. In the temperature control of the oxygen sensor by the heater the temperature of the oxygen sensor is surmised based upon measurement of the electric resistance of the heater, as described in, for example, Japanese Patent Laid-open Publication Nos. 57-197459, 61-35347, 60-202348 and 60-164240. The oxygen sensor is generally a small device having a small heat capacity, and therefore it changes its temperature readily according to the temperature, flow rate, etc. of the exhaust gases with which it is brought into contact. In view of these, it is also known to change electric power supplied to the heater in accordance with operating conditions of the internal combustion engine, as described in, for example, Japanese Patent Laid-open Publication Nos. 60-235050.
The above-mentioned conventional temperature control for the oxygen sensor is based upon the fundamental physical phenomenon that the electric resistance of an electric heater changes in accordance with the temperature thereof. However, the value of electric resistance of the heater incorporated in the oxygen sensor is different within an allowable range of error according to each commercial product. As shown in FIG. 2, although each oxygen sensor of the same design shows a certain determinate rate of change of electric resistance relative to the change of temperature, the absolute value of the temperature at which each oxygen sensor has a certain electric resistance such as Rh.sub.1 differs relatively widely so as to be T.sub.3 -T.sub.4 so that the performance lines for the same products distribute within an upper limit shown by a broken line and a lower limit shown by a dot and dash line.